Nowadays, molten iron is produced by reduction of iron oxide materials such as iron ore by blast furnace-converter processes in the main. This process absolutely requires coke as a reductant. Furthermore, the process makes economics of scale a priority; hence, the process cannot meet limited production of diversified products upon a change in economic trend.
Direct ironmaking processes such as a MIDREX process are suitable for limited production of diversified products. Unfortunately, these processes use natural gas as a reductant; hence, the sites for constructing plants are limited.
Another method for making molten iron is a SL/RN process that includes production of reduced iron with coal-based carbonaceous reductants and melting of the reduced iron in an electric furnace. Many direct ironmaking processes have also been reported in which a rotary hearth furnace and an electric melting furnace are combined for integration of reduction of iron oxide and melting of the reduced iron. Since these processes consume a great deal of electric power, construction of plants is limited to sites to which electric power can be easily supplied.
Under such circumstances, improvements in smelting reduction process that produces molten iron using iron sources such as iron ore and carbonaceous reductants such as coal have been intensively studied. Typical examples of the processes are a DIOS process and a HIsmelt process using a combination of a prereduction furnace and a smelting reduction furnace. Key points in practical use of these processes are a high secondary combustion ratio and high heat transfer efficiency in the smelting reduction furnace. However, in such conditions, slag, which is produced as by-product during smelt reduction from gangue components in iron sources such as iron ore, inevitably contains a high content of iron oxide (FeO). The iron oxide significantly erodes lining refractory of the furnace. A proposed method includes water-cooling of the furnace to suppress the erosion of the refractory. This proposed method, however, has large heat loss from the furnace, significantly reducing the productivity of molten iron and thermal energy efficiency.
One of the direct ironmaking processes includes heating of carbonaceous agglomerates (pellets or briquettes), which are shaped mixtures of iron sources such as iron ore and carbonaceous reductants such as carbonaceous materials, in a rotary furnace to reduce the iron sources and reduction of the product in a smelting reduction furnace. This process introduces hot exhaust gas generated in the smelting reduction furnace into the rotary furnace and uses the heat of the exhaust gas in the smelting reduction furnace to enhance the overall heat efficiency of the facilities. However, the hot exhaust gas from the smelting reduction furnace contains a large amount of dust that is deposited not only on the inner walls of pipes but also on the walls of the rotary furnace, inhibiting a stable continuous operation.
This process has another problem. If thermal fluctuation occurs in the smelting reduction furnace, the heat quantity of the hot gas supplied to the rotary furnace and the reduction potential vary, resulting in an unstable operation of the facilities. Such an unstable operation causes changes in reduction efficiency of iron oxide and metallization in the rotary furnace. As a result, the produced iron does not have constant purity. Furthermore, the byproduct slug contains an increased amount of iron oxide (FeO), which erodes the hearth refractory.
In addition, in the smelting reduction process, large amounts of oxygen and heat are supplied into the smelting reduction furnace. Thus, maintenance of furnace refractory and the tuyere is essential by using equipment for tilting and moving the furnace, resulting in increased production costs of molten iron.